Neoproterozoic Ophiolites of the Arabian-Nubian Shield

34 Pages·2004·1.2 MB·English

PrecambrianOphiolitesandRelatedRocks
EditedbyTimothyM.Kusky
DevelopmentsinPrecambrianGeology,Vol.13(K.C.Condie,SeriesEditor) 95
©2004ElsevierB.V. Allrightsreserved.
Chapter3
NEOPROTEROZOICOPHIOLITESOFTHE
ARABIAN-NUBIAN SHIELD
ROBERT J. STERNa,PETER R. JOHNSONb, ALFRED KRÖNERc AND
BISRAT YIBASd
aGeosciencesDepartment,UniversityofTexasatDallas,Box830688,
Richardson,TX57083-0688,USA
bSaudiGeologicalSurvey,P.O.Box54141,Jiddah21514,SaudiArabia
cInstitutfürGeowissenschaften,UniversitätMainz,55099Mainz,Germany
dPullesHowardanddeLange,EnvironmentalandWaterQualityManagement,
P.O.Box861,AucklandPark2006,SouthAfrica
Ophiolites of mid-Neoproterozoic age are abundant in the Arabian-Nubian Shield
(ANS)ofNEAfricaandArabia.ANSophiolitesrangeinagefrom690to890Maandlitter
aregionthatis3000kmN-Sand>1000kmE-W.InthenorthernANS,ophiolitesoccur
as nappecomplexesmarkingsuturezonesbetween terranes.Althoughdismemberedand
altered,allofthediagnosticcomponentsofophiolitescanbefound:harzburgite,cumulate
ultramafics,layeredaswellashigherlevelgabbroandplagiogranite,sheeteddikes,andpil-
lowedbasalt.Allochthonousmafic-ultramaficcomplexesinthesouthernANS,inEthiopia
andEritrea,areinterpretedasophiolites,butaremoredeformedandmetamorphosedthan
those in the north. Reconstructed ophiolitic successions have crustal thicknesses of 2.5
to5km.
TheANSophioliticmantlewasmostlyharzburgitic,containingmagnesianolivinesand
spinelsthathavecompositionsconsistentwithextensivemelting.Cr#forspinelsinANS
harzburgitesaremostly >60,comparabletospinelsfrommodernforearcsanddistinctly
higherthanspinelsfrommid-oceanridgesandbackarcbasinperidotites.ANSophiolites
areoftenassociatedwithathick(1–3km)sequenceofcumulateultramaficrocks,which
defineatransitionzonebetweenseismicandpetrologicMohos.Thesecumulatesaredom-
inated by dunite, with subordinate pyroxene-richlithologies. Cumulate ultramafics tran-
sition upwards into layered gabbro. Several crystallization sequences are inferred from
ANS transition zones and cumulate gabbro sections. In all samples studied, olivine and
spinel crystallized first, followed (in order of decreasing abundance) by cpx-plag, cpx-
opx-plag,andopx-cpx-plag.ANSophioliticlavasmostlydefineasubalkalinesuitechar-
acterized by low K and moderate Ti contents, that has both tholeiitic and calc-alkaline
affinitiesandincludesasignificant,althoughsubordinate,amountofboninites.Thelavas
are fractionated(mean Mg#=55)but have higherabundancesof Cr (mean=380 ppm)
andNi(mean=135ppm)thanwouldbeexpectedforsuchalowMg#.TheANSophiolitic
lavasincludebothLREE-depletedandLREE-enrichedvarieties,butasagroupareslightly
DOI:10.1016/S0166-2635(04)13003-X
96 Chapter3:NeoproterozoicOphiolitesoftheArabian-NubianShield
LREE-enriched:mean(Ce/Yb) ∼2.2andSm/Nd∼0.30.Onavarietyofdiscrimination
n
diagrams,thelavasplotinfieldsforMORB,BABB,arctholeiite,andboninite.ANSlavas
cluster aroundTi/Zr=97, indicatingthat Ti-bearingphasesdid notprecipitateearly.Nd
isotopiccompositionsindicatederivationfromalong-depletedmantlesource,withεNd(t)
∼+5 to +8.Mineralandlava compositionsareconsistentwiththehypothesisthatmost
ANSophiolitesformedin‘suprasubductionzone’(SSZ)settings,andthehighCr#ofANS
ophioliticharzburgitessuggestsaforearcenvironment.Geochemicalstudiesofdeepwater
sedimentsdepositedon ANS ophiolitesare neededto better characterizeand understand
theNeoproterozoicoceanwhereANSophiolitesformed.
1. INTRODUCTION
The Arabian-NubianShield (ANS) in NE Africa and W. Arabia is the largesttract of
juvenilecontinentalcrustofNeoproterozoicageonEarth(PatchettandChase,2002).This
crustwasgeneratedwhensmallerterranesofarcandbackarccrustweregeneratedwithin
andaroundthemarginsofalargeoceanictractknownastheMozambiqueOcean,which
formedinassociationwiththebreakupofRodinia∼800–900Ma(Stern,1994).Oceanic
plateausmayalsohavebeenaccreted(SteinandGoldstein,1996).Thesecrustalfragments
collided as the Mozambique Ocean closed, forming arc-arc sutures, composite terranes,
theArabian-NubianShield(ANS;Fig.1),andthelargercollisionalbeltknownastheEast
AfricanOrogen(Stern,1994;Kuskyetal.,2003).TheArabian-NubianShieldwascaught
between fragments of East and West Gondwanaland as these collided at about 600 Ma
(Meert, 2003) to form a supercontinent variously referred to as Greater Gondwanaland
(Stern,1994),Pannotia(Dalziel,1997)orjustGondwanaland.TheANSwassubsequently
buriedbyPhanerozoicsedimentsbuthasbeenexposedbyupliftanderosionontheflanks
oftheRedSeainOligoceneandyoungertimes.
SeverallinesofevidencesupporttheideathattheANSisjuvenileNeoproterozoiccrust,
including non-radiogenicinitial Sr and radiogenicinitial Nd isotopic compositions for a
Fig.1. LocationoftheArabian-Nubian Shieldandlocationofophiolitesandrelatedrockswithin
it. (A) Political and modern geographic features of the region. B = Bahrain, Dj = Djibouti,
Is=Israel, Jrdn=Jordan, K =Kuwait. Location of Figs. 2A and B shown in dashed rectan-
gles.(B)LocationofPrecambrianbasementexposures,crustaltypes,andophiolitesandophiolitic
rocks(showninblack).Abbreviationsforsomeofthebetterstudiedophiolitesfollow.SaudiAra-
bia: H=Halaban, JT=Jebel Tays; JU=Jebel Uwayjah; JE=Jebel Ess;AA=Al ‘Ays(Wask);
BT=Bi’rTuluhah;A=Arjah;DZ= DarbZubaydah;BU=Bi’rUmq;Th=Thurwah;JN=Jebel
Nabitah; T=Tathlith. Egypt: F=Fawkhir; Br=Barramiya; Gh=Ghadir; AH=Allaqi-Heiani;
G=Gerf. Sudan: OSH=Onib-Sol Hamed; Hs=Hamisana; AD=Atmur-Delgo; K =Keraf;
R=Rahib; M=Meritri; Os=Oshib; Kb=Kabus (Nuba Mts); I=Ingessana; Kk=Kurmuk.
Eritrea: Hg=Hagar Terrane. Ethiopia: Zg=Zager Belt; DT=Daro Tekli Belt; Bd= Baruda;
TD=TuluDimtu;Y=Yubdo; A=Adola;My=Moyale. Kenya: S=Sekerr,B=Baragoi.The
Bi’rUmq-NakasibsutureisdefinedbytheBi’rUmq-Thurwah-Meritri-Oshibophiolitebelt.
1. Introduction 97
)
B
(
)
A
(
98 Chapter3:NeoproterozoicOphiolitesoftheArabian-NubianShield
wide range of igneous rocks. The ANS can be isotopically defined as the region in NE
Africa and Arabia where Nd-modelages approximate crystallization ages (Stern, 2002).
These indications that abundant juvenile continental crust and mantle lithosphere were
generated during Neoproterozoic time in the region has been confirmed by Nd and Sr
isotopic studies of samples of mafic lower crust and mantle lithosphere brought up as
xenolithsinTertiarylavasfromSaudiArabia,whichalsoindicatethatthelowercrustand
lithosphericmantleoftheregionformedduringNeoproterozoictime(Henjes-Kunstetal.,
1994;McGuireandStern,1993).
OphiolitesandophioliticrocksareremarkablyabundantintheArabian-NubianShield.
They are scattered across most of the ANS, over a distance of ∼ 3000 km from the
farthest north (Jebel Ess) almost to the equator, and from Rahib in the west to Jabal
◦
Uwayjah (45 E) in the east, encompassing an area of about two million square kilome-
ters (Fig. 1). Ophiolitesare particularly well studied in Arabia (see companionpaper by
Johnsonetal.(2004). If ophiolites are the remains of oceanic lithosphere, then the ANS
is a massive graveyard of Neoproterozoicoceanic lithosphere. The abundance of ophio-
lites is a further indication that ANS crust and lithosphere were produced by processes
similartothoseofmodernplatetectonics.Theirabundancehasmadeitdifficulttodefine
the orientation of sutures solely from the distribution of ophiolitic rocks (Church, 1988;
Sternetal.,1990).ThisisfurthercomplicatedbythefactthatnotallANSmafic-ultramafic
complexes formed in a seafloor spreading environment—some appear to be roots of is-
land arcs, such as Darb Zubaydah in Arabia (Quick and Bosch, 1989)—and others are
autochthonouslayeredintrusions,suchasDahanibinEgypt(Dixon,1981).Caremustbe
exertedto avoid misidentification,but even the most conservativeestimates indicate that
thereisaremarkableabundanceofophiolitesintheANS.
OphioliteswerefirstrecognizedintheregionbyRittmann(1958),butwereotherwise
ignoreduntilthepioneeringstudyofBakoretal.(1976).Thiswasfollowedbyaflurryof
field,petrological,andgeochemicalstudiesthroughoutthe1980’s.Thislevelofactivityhas
decreasedsignificantlythroughthe1990’sandintothe21stcentury.Thisreviewhasthree
objectives.First, itisintendedtosummarizethe mostimportantobservationsofthisfirst
phaseofstudyingANSophiolites.Second,becausetheophiolitesoftheArabian-Nubian
Shield are so commonand so well-exposed, it is hoped that this example will provide a
basisofwhatisexpectedtobepreservedwhenamajorepisodeofjuvenilecrustformation
associatedwithmodern-styleplatetectonicsoccurs.Finally,itishopedthatthisoverview
willstimulatearesurgenceofANSophiolitestudies.
Note that the Arabic word for mountain is variously spelled ‘Jebel’ (Egypt), ‘Gebel’
(Sudan), or ‘Jabal’ (Arabia). For simplicity, we use the ‘Jebel’ spelling throughout this
contribution.
2. OUTCROPPATTERNS
We define the ANS as the northern, juvenile part of the Neoproterozoic EAO. More
restrictivedefinitionshavetheshieldendinginthesouthwiththesouthernmostcontiguous
3. CrustalStructure 99
outcropsofbasementexposedaroundtheRedSea.TheArabian-NubianShieldasdefined
hereincludesbasementoutlierswelltothesouthinEthiopiaandKenya.Forthepurposesof
thispresentation,theANSisthatpartoftheEastAfricanOrogencharacterizedbyjuvenile
Neoproterozoic crust and is also where ophiolites and ophiolitic rocks are encountered
(Fig.1).
The ANS can be usefully subdivided into northernand southern halves. North of the
Bi’r Umq-Nakasib suture, which extends NE from the Oshib and Meritri ophiolites in
Sudan and continues across the Red Sea in Arabia through the Thurwah and Bi’r Umq
ophiolites, the ophiolite belts trend approximately E-W. It is relatively easy to identify
structures developed during ophiolite obduction in this region. Greenschist-facies meta-
morphismis characteristicfor these ophiolites,and diagnosticfeatures, such aspillowed
basalts,arewellpreserved.ThisisshowninTable1,whichlistsophioliteswithaPenrose-
type succession;these are fromthe northernANSor fromthe Bi’r Umq-Nakasibsuture.
Suchexcellentpreservationlargelyreflectsthefactthatthesenorthernophiolitesarerela-
tivelyundisturbedbysteepN-SstructuresdevelopedduringterminalcollisionbetweenE
andWGondwanaland,whicharepervasivefarthersouth(Fig.2A).Manyophiolitesinthe
northernANS are, however,disrupted by NW-trendingstrike-slip faults and shear zones
ofthe Najdfaultsystem(Sultanetal., 1988).Remotesensinghasprovento bean effec-
tive way to mapthe distributionof spectrallydistinctlithologies,especiallyserpentinites
andamphibole-bearingmaficrocks,providingaquantitative,ifindirect,assessmentofthe
distributionandabundanceofdisruptedophiolitesinthebasementofEgypt(Sultanetal.,
1986).AmoredetailedpresentationofthefieldrelationsandstructureofArabianShield
ophiolitesispresentedbyJohnsonetal.(2004).
It is a significantly greater challenge to identify ophiolites to the south of the Bi’r
Umq-Nakasibsuture.Thisareawasclosertoandthusmoreintenselyaffectedbytheend-
Neoproterozoicterminalcollision, such that structures related to ophiolite obductionare
transposed or obliterated (Abdelsalamand Stern, 1996).Basement structuresdip steeply
(Fig. 2B), units are intensely deformed and shuffled by high-angle thrusting and sub-
horizontal shearing, and metamorphism is typically amphibolite-facies (Yihunie, 2002).
Diagnosticfeaturesandemplacementfabricsforunitsthatmightoriginallyhavebeenophi-
olitesarenotcommon.Puristswouldhesitatetoidentifythelinearmafic-ultramaficcom-
plexes of the southern ANS as ophiolites, but the association of harzburgiticultramafics
in associationwith MORB-likeandevenboniniticmaficunitsaswellasthe regionalas-
sociation of southern ANS mafic-ultramafic complexesto the abundantand unequivocal
ophiolites of the northern ANS makes it likely that these mostly represent ophiolites in
differentstagesofpreservation.
3. CRUSTALSTRUCTURE
The best preserved ophiolites in the northern ANS contain all or most of the com-
ponents of complete ‘Penrose’ ophiolites (Table 1), including pillowed basalts, gabbros,
100 Chapter3:NeoproterozoicOphiolitesoftheArabian-NubianShield
Table1. ANSophioliteswitha“Penrose-type”assemblage
Name Country Reference
DarbZubaydah SaudiArabia Quick(1990)
Bi’rTuluhah SaudiArabia Pallisteretal.(1988)
WadiKhadra SaudiArabia Quick(1991)
Halaban SaudiArabia Al-Salehetal.(1998)
Ess SaudiArabia Pallisteretal.(1988)
Al‘Ays(Wask) SaudiArabia Bakoretal.(1976)
Tharwah SaudiArabia Nassiefetal.(1984)
Bi’rUmq SaudiArabia Shanti(1983)
Tathlith SaudiArabia Pallisteretal.(1988)
Oshib-Ariabbelt Sudan Abdel-Rahman(1993)
Arbaat Sudan AbdelsalamandStern(1993)
Atmur-Delgo Sudan Harmsetal.(1994),
Schandelmeieretal.(1994)
SolHamed Sudan Fitchesetal.(1983)
WadiOnib Sudan Husseinetal.(1983)
GebelGerf Egypt Zimmeretal.(1995)
WadiGhadir Egypt El-Bayoumi(1983)
Fawkhir Egypt El-Sayedetal.(1999)
andtectonizedharzburgites.SeveralANSophioliteshavesheeteddikecomplexes(suchas
Ghadir,Onib,andEss)butthesearenotalwaysreported.Eventhewell-preservedophio-
litesarefaulted,folded,andotherwisedisrupted,sothatreconstructingacompleteophio-
litepseudostratigraphyisdifficultandequivocal.Nevertheless,threesuchreconstructions
ofANSophiolitecrustalstructureareshownonFig.3.Thesereconstructionsdifferinthe
relativeabundancesofvolcanics,pillowedlavas,sheeteddikes,andgabbrobutallsuggest
that the oceanic crust represented by ANS ophiolites was generally in the range of 2.5
to 6 km thick.As discussed in the nextsection, ANS ophioliteswere generatedandem-
Fig.2. RemotesensingimagesofANSophiolites,showingthedifferentoutcroppatternsofophi-
olites in the northern (A) and southern (B) parts of the Arabian-Nubian Shield. Locations shown
inFig.1.(A)Allaqi-HeianiSuturealongtheEgypt-Sudanborder.Imageisapproximately300km
acrossandNistowardsthetopofimage.DashedlineapproximatestraceofAllaqi-HeianiSuture.
Notethat thegeneral E-Wstructureof theophiolitebelt,whichformed duringsuturingof theSE
DesertandGabgabaterranesisonlydisruptedbyyoungerN-Sstructures(developedduringterminal
collision between East and West Gondwanaland) of the N-S Hamisana Shear Zone. This outcrop
patternindicatestheophiolitesandassociatedaccretionarystructuresaresubhorizontal.NASAas-
tronaut photograph (S32-74-100). (B) Landsat TM image of basement units in N. Eritrea and E.
Sudan,showingdominanceofcomplexdeformationrelatedtoterminalcollisionbetweenEastand
West Gondwanaland, resulting in ∼ N-S structures. Terrane names are modified after Drury and
Filho(1998).Sceneisabout90kmacross.
3. CrustalStructure 101
102 Chapter3:NeoproterozoicOphiolitesoftheArabian-NubianShield
Fig.3. ReconstructedcrustalsectionsofANSophiolites.Al‘Ays(AlWask),SaudiArabia(Bakor
etal.,1976); Bi’rUmq,SaudiArabia(Al-RehailiandWarden,1980); Onib,Sudan(Kröneretal.,
1987).
placedrelativelyearlyinthehistoryoftheANSandEAO.Fragmentsfromdismembered
ophiolitesarecommonintheANS,anditbecomesmoredifficulttointerprettheseasonce
beingallochthonouspiecesofoceaniccrustasthesefragmentsbecomemoredeformedand
metamorphosed.Sufficeittosaythatnotallultramaficrocksintheshieldare—orwere—
parts of ophiolites. There are many layered igneous intrusions containing non-ophiolitic
ultramafics and gabbros and there are many examples of non-ophiolitic pillowed lavas.
Nevertheless, the association of harzburgitic ultramafics and low-K tholeiitic metabasalt
arguesstronglythattheseoncebelongedtoacoherentophiolite.
4. Age 103
Fig.4. AgesofophiolitesintheArabian-NubianShield,binnedat25millionyearintervals.Neo-
proterozoic time (544 to 1000 Ma) is subdivided into Tonian (1000–850 Ma), Cryogenic (850 to
∼600 Ma) and Neoproterozoic III (∼600–544 Ma), after Knoll (2000). Age range when Ara-
bian-NubianShieldwastectonicallyandmagmaticallyactive(870MatoendofNeoproterozoic)is
alsogiven.OphioliteagesareU-PbandPb-PbzirconagesandSm-Ndagesfrom(Staceyetal.,1984;
Claessonetal.,1984;Pallisteretal.,1988;Kröneretal.,1992;Zimmeretal.,1995;Worku,1996).
Meanageforophiolites±1standarddeviationisgiven.
4. AGE
ANS ophioliteshave been reliably dated using U-Pb zircon techniques(Stacey et al.,
1984;Pallisteretal.,1988)andPb-Pbzirconevaporationtechniques(Kröneretal.,1992;
Zimmer et al., 1995) on zircons separated from gabbros and plagiogranites. Other ages
have been generated using Sm-Nd mineral and whole-rock techniques (Claesson et al.,
1984; Zimmer et al., 1995;Worku, 1996).These results give age ranges of 694±8 Ma
fortheyoungestANSophiolite(Urd/Halaban;Staceyetal.,1984)to870±11Maforthe
oldest(Thurwah;Pallisteretal.,1988).Ameanof781Ma(1standarddeviation=47Ma)
is obtained for 16 robust ophiolite ages (Fig. 4). Ophiolites formed during the first half
ofthetimeperiodencompassedbytectonicandmagmaticactivityoftheArabian-Nubian
Shield.Thereis noobviouslysystematicgeographicvariationinthe distributionofANS
104 Chapter3:NeoproterozoicOphiolitesoftheArabian-NubianShield
ophioliteages;theoldestophioliteisinthemiddleoftheANSandtheyoungestisnearthe
easternedgeofANSexposures.
5. OPHIOLITECOMPONENTS
5.1. Harzburgites
Where the original protolith can be identified, the mantle peridotites associated with
ANSophiolitesarepredominantlytectonizedharzburgites,lherzolitebeingrarelyreported.
StudiesofANSophiolitefabricsareataveryearlystage,butharzburgitesanddunitesof
the Thurwah ophiolite have been tectonized at high temperatures (Nassief et al., 1984),
and a similar high-temperature ductile fabric is reported from the Sol Hamed ophiolite
(Fitchesetal.,1983). The harzburgitesrepresent residual mantle after extensive melting,
whereasthedunitesandwehrlitesarecumulatesorreflectmelt-wallrockinteractions.
Harzburgites are mostly altered (Figs. 5A, B), but relict olivines and pyroxenes have
been analyzed by electron microprobe for 5 ophiolites. Harzburgite associated with the
AlAysophiolitecontainsolivineofFo ,orthopyroxeneofEn ,andclinopyroxene
91–94 89–91
of En , Fs , Wo (Chevremontand Johan, 1982a;Ledru and Auge, 1984).
48–53 2.2–3.0 44–49.2
HarzburgitefromthenearbyHwanetophiolitehasorthopyroxeneofEn andclinopy-
88.9–91
roxene of En , Fs , Wo (Chevremontand Johan, 1982a). Harzburgite
48.9–50.3 2.3–3.4 46.3–48.9
associated with the Ess ophiolite contains olivine of Fo , orthopyroxeneof En ,
91–93 88–92
anddiopsidicclinopyroxeneofEn ,Fs ,Wo (Al-Shanti,1982).Nassiefet
49–52 2.4–3.1 44.6–48.3
al.(1984)identifiedamantlesequencethatisupto20kmthickfortheThurwahophiolite,
and harzburgite from this consists of 70–90% olivine (Fo ), 15–30% orthopyrox-
89.5–93.4
ene (En ) and <1% each of chromite and clinopyroxene.Mouhamed(1995)argued
90–92
onthebasisofCIPWnormativecompositionsofMuqsimserpentinitesalongtheAllaqi-
Heianisuture(Fig.2)thatthesewereoriginallyharzburgites.OlivinesfromtheIngessana
ophioliteareFo (Price,1984).Thesecompositionsareatthemagnesium-richendof
91–97
peridotites,asshownonFig.6.
Olivine compositions provide insights into the tectonic setting of ophiolites because
magmagenesisin differenttectonicsettingsreflectsdifferingextentsofmelting.Because
residual olivines become increasingly magnesian as melting progresses, residual mantle
shouldhaveolivinesthataremoremagnesianthantheFo ofundepleted‘pyrolitic’up-
88
permantle(Fig.6).BonattiandMichael(1989)suggestthatmantlemeltingrangesfrom
nearlyzeroforundepletedcontinentalperidotitestoabout10–15%meltingforriftedmar-
gins to 10–25% melting associated with mid-ocean ridge (MOR) peridotites to 30% for
peridotites recovered from forearcs, which generally form during the early stages in the
evolutionoftheassociatedsubductionzone(Bloomeretal.,1995).Mantleperidotitesfrom
back-arcbasinswerenotavailablewhenthisdiagramwasoriginallygenerated,butsince
thattimemantleperidotitesfromtheMarianaTroughactiveback-arcbasininthewestern
Pacific have been studied(Ohara etal., 2002).These harzburgiteshave olivinecomposi-
tionsthatareindistinguishablefromMORharzburgitesandaredistinctlylessmagnesian